1. Field of the Invention
The present invention relates to the field of networking and more specifically to provide information over a cable modem network.
2. Description of the Related Art
In a cable modem network, information is carried over a cable from a cable modem termination system (CMTS) to a plurality of cable modems (CMs). The CMs are coupled in communication with the CMTS to receive information on a so-called "downstream" channel and to communicate information to the CMTS on a so-called "upstream" channel. Particular characteristics (e.g., frequency, power levels, etc,) of the upstream channel are determined at the time the CM is initialized.
FIG. 5 illustrates an implementation in which a single downstream channel 501 is utilized in conjunction with four upstream channels 511-514. The four upstream channels 511-514 may be, for example, separate fibers serving four geographically separate communities of CMs 521-524. The Radio Frequency Interface Specification, Data-over-cable Service Interface Specifications, SP-RFI-I03-980202 available from the Cable Television Laboratories, Inc. (hereinafter, the "MCNS specification") describes a basic initialization process for CMs. Initialization of CMs in an implementation as shown in FIG. 5 is described in greater detail in Appendix G of the MCNS specification.
As is discussed in the MCNS specification, the CMTS 531 periodically broadcasts upstream channel descriptors (UCDs) on the downstream channel 501 in order to describe characteristics of upstream channels to the CMs. As part of the initialization process, a CM waits for a UCD in order to obtain transmission parameters for the upstream channel. When the CM receives a UCD and determines that the transmission parameters are suitable, it waits for a SYNC message from the CMTS in order to achieve time synchronization with the CMTS. It then waits for a bandwidth allocation map for the selected channel. The allocation map allocates periods in the bandwidth to various types of data transmissions. One of these may be an "initial maintenance" request. It is during this initial maintenance request time slot that a new CM may transmit what is termed a "ranging request" which is used to determine network delay. In response to the ranging request, the CMTS transmits a "ranging response" which, may among other parameters, include a parameter redirecting the CM to an appropriate upstream channel. As is discussed in Appendix G to the MCNS specification, this topology imposes a number of constraints including:
All of the upstream channels 511-514 must operate at the same frequency because, as a result of the CM essentially choosing a UCD at random (typically, the first UCD received during the initialization cycle), it would be transmitting at the wrong frequency if this constraint were not imposed. PA1 All of the upstream channels 511-514 must operate at the same symbol rate because the CMTS would otherwise be not be aware of the symbol rate of the transmissions and would be unable to demodulate the ranging requests; PA1 All of the initial maintenance opportunities must be aligned because the CMTS must be prepared to receive a ranging request at the time designated in the allocation map; and PA1 All initial maintenance opportunities must use the same burst characteristics so that the CMTS can demodulate the ranging request.
These constraints raise a number of issues. For example, when there is excessive ingress (noise and RF interference) on a particular channel, it is desirable to change the frequency of the channel. However, it may be that the ingress is only occurring on one of channels 511-514 at a given frequency because of particular characteristics (environmental, etc.) of that channel. However, as a result of the first constraint, if the frequency of one channel is changed, the frequency of all channels must be changed. Thus, not only is the overhead of needing to change the frequency of multiple upstream channels imposed by this constraint, but the likelihood of experiencing ingress at particular frequency is increased because ingress on any channel requires the making a change.
Thus, what is needed is a method and apparatus relieving as many as of the above-recited constraints as possible. It would be desirable if such a method and apparatus allowed operation of a cable modem plant in accordance with the MCNS specification.